Jumper cables would be efficient, in that little power would be lost in them, but the tug's system voltage will be what the tug needs... not necessarily what the trailer battery needs. If the trailer battery is very low and you need more in it, that would be fine; if you really are trying to "top off" the trailer battery, that may not happen.

Jumper cables would be efficient, in that little power would be lost in them, but the tug's system voltage will be what the tug needs... not necessarily what the trailer battery needs. If the trailer battery is very low and you need more in it, that would be fine; if you really are trying to "top off" the trailer battery, that may not happen.

That's true. You can't get blood out of a turnip.

__________________
"You can't buy happiness, but you can buy an RV. And that is pretty close."

I used my Tacoma's 400 watt tailgate plug to top up the two 12 volt batteries in my trailer by using an extension cord. With my Tacoma idling, my batteries topped up to 100 percent with 13.6 amps (as seen on my Go Power meter). The fridge went directly to AC mode while on tailgate power. We spent 10 days under a rather thick canopy of trees so the solar panel was little to no good. Running the Tacoma for an hour, I must admit, is less fuel efficient than a Honda 2000 generator but I believe the truck is quieter than a generator and I saved myself $1300.00.

I used my Tacoma's 400 watt tailgate plug to top up the two 12 volt batteries in my trailer by using an extension cord. With my Tacoma idling, my batteries topped up to 100 percent with 13.6 amps (as seen on my Go Power meter).

The 13.6 is volts. It is the voltage that the batteries are charged by. If you put a voltmeter on your trucks cigarette lighter socket while it was running it woud be the same. (within a couple of tenths of a volt.)
You can simply run an extension from your truck cigarette lighter socket to a trailer cigarette lighter or make a cable that plugs into your cigarette lighter at one end and a female socket at the other end to attach to your trailers power/brakes plug.
My solar panel was never installed onto my camper for the reason that, yes, we try to park in the shade. I had it on a 40 foot cable so that I could put it in the sun. It was a 50 watt panel and easily topped up my batteries every day. Except in winter it could not keep up with the furnace usage.
The fan in the furnace used an inordinate amount of power (amperage) compared to any other fan I've seen of that size.
If you are going to run your truck I believe the 12volt to 12volt is the most efficient